In the previous funding period we identified immunogenetic loci that an unrelated donor (URD) KIR B haplotype yields statistically significant and meaningful improvement in protection against relapse and relapse free-survival fpr patients with AML. New analyses have refined the favprable KIR B loci to those which encode donor KIR activating receptors associated with improved clinical outcome. We will further explore the clinical importance of these genetic loci by prospectively evaluating the logistics and clinical impact of donor selection by KIR genotyping for URD hematopoietic cell transplantation (HCT) in AML. We hypothesize that amongst URD with optimal HLA matching, donor KIR genotyping can identify better donors likely to yield improved clinical outcome. In conjunction Project 1, we will explore the functional significance of these KIR B loci, their interaction with HLA Class I and allelic polymorphism in the KIR regions to further refine our understanding and methods for prospective donor selection. With Project 2 we will analyze NK cell functional development over time after URD HCT and correlation of this NK development with the complications following HCT including engraftment, GVHD, infections, relapse and survival. These unique analyses accompany a BMT CTN prospective trial testing blood vs. marrow as a graft source for URD HCT and offer a unique platform for understanding how NK cell development, KIR genotyping and functional immune reconstitution can modify post-transplant complications and outcomes. Additionally, the fundamentals of NK cell therapy to reduce recurrence of resistant leukemia and improve sun/ival after transplantation will.be directly tested. In a multicenter trial of reduced intensity haploidentical HCT plus donor NK infusions for resistant AML we will evaluate the direct impact of NK cell therapy on highly resistant leukemia. Prospective clinical trials will define elements essential for safer application of this treatment. Overall, these studies can improve the survival and relapse protection following allogeneic HCT for AML. Our studies outline new and exciting opportunities to improve donor selection and maximize the safety and effectiveness of allotransplantation.